Gain information about the origin and early differentiaion of lymphoid stem cells in the chicken by transplantation of stem cells or allogeneically educated, more mature cells into immunodeficient chicks or chicken embryos. Studying the fate and function of transplanted cells by morphological and functional means and studying the origin of prebursal and prethymic stem cells by transplantation of 48-hour chicken embryos on a foreign yolk sac to determine whether stem cells originate in the yolk sac or intra-embryonically.